Standard Storm Atronachs (not summoned ones) and The Sunken One are the only sources of this ingredient. Storm Atronachs are commonly encountered in Planes of Oblivion at levels 19 and higher. As with other daedric creatures, in Tamriel the best place to look for Storm Atronachs is in Conjurer Dungeons (circles on map), where they may spawn at levels 19 and higher.

Trying to find creatures with Void Salts before level 19 is challenging. Only one low-level source exists:

The Sunken One appears in Sandstone Cavern (red circle) at all levels. It spawns during the quest The Sunken One and will respawn after the quest is completed.

Other sources are primarily restricted to locations specific to the Main Quest.

During the quest The Battle for Castle Kvatch, there are 9 courtyard creatures and 19 town creatures who may be Storm Atronachs starting at level 18.